The present invention relates to a process for producing a metal alloy powder containing nickel and/or cobalt, chromium, aluminium and yttrium in the form of a xcex3 phase and a xcex2 phase dispersed in the xcex3 phase, capable of forming from the xcex2 phase, by exposure to air at elevated temperatures, an adhering superficial alumina barrier.
It is known to protect metal parts operating at high temperatures, in particular aeronautical turbine blades, against corrosion and/or oxidation by applying various coatings obtained by diffusion (mainly based on nickel aluminide xcex2-NiAl, if necessary modified by additional elements). These coatings have numerous advantages but are restricted as regards composition. For certain applications the xe2x80x9cidealxe2x80x9d coating may have a chemical composition that is impossible to obtain by diffusion. This is why the projection application of alloys such as those described by the qualitative formula MCrAlY (where M=Ni and/or Co and/or Fe) has been studied by means of physical deposition techniques. These techniques enable so-called xe2x80x9cactivexe2x80x9d elements such as yttrium, hafnium, tantalum and zirconium to be added in trace amounts (up to 2%) to the deposited coating.
A typical example of an MCrAlY coating consists of a nickel-based alloy containing 20% of cobalt powder, enabling the following reaction to be avoided:
xcex3-Ni+xcex2-NiAlxe2x86x92xcex3xe2x80x2-Ni3Al+xcex1-Cr,
which is possible above 1,000xc2x0 C., 20 to 25% of chromium in order to reinforce the resistance to type I corrosion, 6 to 8% of aluminium (aluminoforming compound) and about 0.5% of yttrium, which reinforces the adherence of the alumina layer to the aluminoforming alloy. Its general microstructure is that of a two-phase alloy containing precipitated xcex2-NiAL (aluminoforming phase) in a xcex3 matrix. According to the conditions of use, other elements may be added and/or the above concentrations may be altered. For example, if the coating is intended to be used to prevent type II hot corrosion in the presence of vanadium, the concentration of chromium may exceed 30% by weight. Numerous MCrAlY compositions are commercially available, the most commonly used being those known under the names AMDRY 997 (NiCoCrAlYTa) and AMDRY 995 (CoNiCrAlYTa).
A particularly interesting feature of these protective alloys is the possibility of adding active elements thereto. The addition, in small amounts (of the order of 1 atomic % or less), of elements such as Y at Hf improves very significantly the adhesion of Cr2O3 or Al2O3 layers to the alloys in question. Due to this, the protective effect of the oxide layer is preserved over prolonged periods, particularly under thermal cycling conditions.
Among the physical deposition techniques noted above for obtaining MCrAlY coatings, there may be mentioned in particular hot projection and more especially plasma projection, in which the material to be deposited is introduced, by means of a carrier gas, into the jet of a plasma torch in the form of powder granules 20 to 100 xcexcm in diameter. After having melted, the droplets of the material that has liquefied are projected at high speed onto the surface of the substrate. The plasma flame is produced by the very rapid expansion in a nozzle anode of a plasma-forming gas (Ar+10% H2 for example) ionised during its passage through an arc chamber. Any material available in powder form that can be melted without decomposing or evaporating can thus be deposited on the surface of a substrate. This projection deposition may take place either at atmospheric pressure (in air or in a neutral atmosphere) or under reduced pressure. In all cases the coating is formed at a high rate, typically at a rate of 100 xcexcm/minute. This deposition technique is extremely directional and is thus difficult to employ with parts that are of complex shape.
It is mainly plasma projection under reduced pressure that is used for the deposition of MCrAlY type alloys. The projection device is installed in an enclosure that is subjected to a high reduced pressure (P=0.05 bar). This enables oxidation of the projected alloy particles to be avoided, increases the velocity of the gases in the plasma jet and elongates the flame, which in turn increases the impact velocity of the molten particles and, consequently, reduces the porosity. Finally, it should be noted that this technique permits an initial ionic pickling to be carried out by polarising the surface of the substrate, which improves the adhesion of the coating to the substrate. The deposits obtained are adherent and slightly brittle and may be very thick (several millimetres thick). After projection of the particles, the MCrAlY coatings are diffused during a thermal treatment in vacuo. They are however rough and require a post-operative machining followed by a tribo-finishing.
There may also be mentioned a high velocity flame projection technique carried out by reacting a fuel (hydrocarbon and/or hydrogen) and a combustion-supporting medium (air reconstituted from a mixture of nitrogen and oxygen, or pure oxygen).
Another category of physical deposition techniques is that of vapour phase physical deposition, which involves triode cathodic sputtering and evaporation in an electron beam.
For the triode cathodic sputtering a three-electrode system polarised to a value of several kilovolts and placed in an enclosure subjected to a vacuum of about 10xe2x88x922 Pa enables extremely adherent MCrAlY alloys that are non-porous and less directional than in the case of plasma projection to be deposited at a rate between 5 and 25 xcexcm/hour.
In order to effect evaporation in an electron beam in an enclosure maintained under a vacuum harder than 10xe2x88x924 Pa, an electron beam is focussed on the surface of the material to be deposited contained in a cooled metal crucible. A continuous ingot feed system enables the level of the liquid bath and the deposition conditions to be maintained constant. The emitted vapours condense on the substrate arranged opposite the liquid bath. This substrate is maintained at a sufficiently high temperature so as to minimise the inherent defects in the columnar growth of the deposit. These defects are subsequently eliminated by shock blasting followed by a thermal diffusion treatment and elimination of stresses. The deposition rates may reach values of up to 25 xcexcm/min. This technique is described in U.S. Pat. No. 5,698,273 A, and was mainly developed for the deposition of MCrAlY on turbine machinery blades. Its widespread use is however limited on account of the associated investment and operating costs. Furthermore, this process is extremely directional and does not enable certain coating compositions to be easily obtained (e.g. in the case of alloys containing elements with widely differing vapour pressures).
On the other hand this process enables a combined protective +thermal barrier coating to be produced (zirconium oxide partially stabilised with yttrium oxide (ZrO2+8% Y2O3)), this combined coating having better mechanical properties and a better resistance to thermal shock than coatings obtained by plasma projection.
Electrolytic deposition of the alloy MCrAlY(Ta) is impossible since it would involve the combined deposition in aqueous medium (water is no longer stable beyond xe2x88x921 V with respect to a normal hydrogen electrode) of nickel (EO=xe2x88x920.44 V), cobalt (EO =xe2x88x920.28 V), chromium (EO=xe2x88x920.744 V), aluminium (EO=xe2x88x921.662 V), yttrium (EO=xe2x88x922.372 V) and, possibly, tantalum (EO=xe2x88x920.750 V). In order to obtain an MCrAlY deposit electrochemically it would thus be necessary to produce a composite deposit comprising on the one hand nickel and/or cobalt, and on the other hand particles of CrAlY, and then to effect the diffusion of the composite consisting of electrolytic deposit+particles+substrate at high temperature (typically 2 hours at 1100xc2x0 C.). Examples of implementation of the above are described in U.S. Pat. Nos. 4,305, 792 A, 4,810, 334 A and 5,037,513 A.
In the absence of an electrolytic deposit, it is possible to cause particles to migrate in a strong electric field (typically 100 V). The chosen deposition medium should have a high dielectric constant and exhibit a high electrochemical stability. Electrophoresis satisfies these criteria. This technique is currently used for painting car bodies and enables metallic or non-metallic particles to be deposited. However, the resultant deposit is porous, friable and not particularly adherent, and has to be reinforced by a second deposit involving a conventional technique.
If the MCrAlY coating is exposed to an oxidising environment, its xcex2 phase oxidises (and consequently becomes depleted in aluminium) to form a stable and impermeable layer of alumina. The amount of xcex2 phase available decreases as this aluminium is consumed, and disappears completely over the course of several hundred hours. From then on the protection of the coating is ensured by the impermeability of the resultant alumina coating with respect to oxygen and by its anchorage in the protective coating. If however this ceramic layer were to disappear accidentally, the coating would no longer be able to re-form it.
In the case of coatings obtained by composite electrolytic deposition, it should be noted that the matrix has a limited composition, namely nickel, cobalt or nickel-cobalt. The additives should then be introduced into the co-deposited powder. This operation requires the preparation of a new alloy and sputtering of the latter, which involves a long series of complicated and expensive operations. The consolidation of the electrophoretic deposits is effected by means of a high temperature vapour phase deposition or by fusion of one of the deposited powders. On account of this fact the flexibility of the composition of the coating will depend on the amount of aluminium permitted in the MCrAlY alloy (xcex2-xcex3 equilibrium) or on the composition of the brazing powder that is used.
In the case of deposits formed by plasma or flame projection, this composition basically depends on the composition of the powder that is used. The changes in composition then necessitate the investigation and production of a new batch of powder, an operation which, as is known, is long and complicated.
Finally, in the case where the coating has to support a thermal barrier, the known processes are unable to promote the adherence of this barrier to the oxide layer (Al2O3) formed on the surface of the MCrAlY.
The object of the invention is to alleviate all or some of the aforementioned disadvantages.
The invention provides in particular a process of the type defined in the introduction, and comprises a stage involving the formation, on a powder of a precursor alloy containing at least one of the elements Cr, Al and Y and using a chemical or electrolytic deposition bath, of a deposit containing at least one modifying element capable of extending the existence domain of the said xcex2 phase and/or of increasing the fineness of its dispersion.
The powder that is thus obtained may be used in particular, as will be seen in more detail hereinafter, to form a protective coating of the MCrAlY type. The extension of the existence domain of the xcex2 phase due to the modifying element enables the said phase to subsist after a high consumption of aluminium and consequently to regenerate if necessary the alumina layer. A finer dispersion of this same phase improves the anchorage of the alumina layer in the MCrAlY coating.
Optional, complementary or alternative features of the invention are listed hereinafter:
The modifying element is selected from platinum, palladium, ruthenium, rhodium, osmium, iridium, iron, manganese and rhenium.
The said bath is an autocatalytic chemical bath containing oxalate ions, ions of the modifying element, and a complexing agent for the latter.
The modifying element is chosen from platinum and palladium, the said complexing agent is ethylenediamine, and the bath is strongly basic and contains in addition at least one stabiliser, hydrazine being added progressively as reducing agent.
The said bath is an aqueous autocatalytic chemical bath of pH between 8.5 and 14, whose dissolved species have the following initial composition in moles/litre:
xe2x80x83the amount of ethylenediamine being sufficient to complex all the palladium ions and to combine moreover with chloride ions, thereby preventing the presence of free chloride ions in the vicinity of the particles to be coated.
The modifying element is deposited, at least in part, in the form of particles of this element or of a compound of the latter suspended in a deposition solution, the said particles being included in a matrix deposited from ions contained in the solution.
The said compound is a silicide.
The deposition solution contains nickel and/or cobalt ions, a compound of a flux element selected from boron and phosphorus and that is introduced progressively as reducing agent, and the matrix contains nickel and/or cobalt combined with the flux element.
The deposition solution contains furthermore at least one complexing agent for the said nickel and/or cobalt ions, and at least one water-soluble organic stabiliser that does not contain either sulfur or any metal or metalloid of Groups IIIa (except boron and aluminium), IVa (except carbon), Va (except nitrogen and phosphorus), VIa (except oxygen) and VIIa (except fluorine and chlorine), and that contains an electron pair that can easily be captured by nickel and/or cobalt.
The deposition stage of the modifying element is preceded or followed by a complementary stage consisting of the deposition of nickel and/or cobalt on a powder containing at least the elements Cr, Al and Y, from an autocatalytic deposition bath containing nickel and/or cobalt ions, the resultant powder from the first of the said stages serving as substrate for the second stage.
In the complementary stage the said bath contains nickel-II hydroxide/tri(ethylenediamine) and/or cobalt-II hydroxide/tri(ethylenediamine) and at least one stabiliser, the ethylenediamine acting as complexing agent and being progressively introduced as reducing agent.
In the deposition stage of the modifying element and if necessary in the complementary stage, the powder to be treated is suspended, while stirring, in the deposition bath.
The stirred suspension is contained in a receptacle whose wall that is in contact with the suspension is substantially spherical.
The object of the invention is also a metal alloy powder such as may be obtained by the process defined above, which includes a powder effectively obtained by this process as well as a powder obtained by a different route but having the same characteristics as the previous powder. In particular, such a powder contains nickel and/or cobalt, chromium, aluminium and yttrium in the form of a xcex3 phase and a xcex2 phase dispersed in the xcex3 phase, capable of forming from the xcex2 phase, by exposure to air at elevated temperature, an adherent superficial barrier of alumina, and contains moreover at least one modifying element capable of extending the existence domain of the said xcex2 phase and/or of increasing the fineness of its dispersion.
The invention also envisages the use of the aforementioned powder to form a coating based on the said alloy on a metal substrate.
The use according to the invention may comprise at least some of the following features:
The granules of the said powder are melted and the resultant droplets are projected by means of a plasma torch onto the substrate to form the coating.
The said coating is formed by electrophoresis in a medium containing the said powder in suspension and also containing an adhesion agent capable of permitting the agglomeration of the powder granules deposited on the substrate, following which a consolidation treatment of the coating is carried out.
The said consolidation treatment comprises the melting of the composite deposit present on the powder granules.
At least one film is formed by presintering the said powder by melting the composite deposit present on the powder granules, the said film is applied to the said substrate with the interpositioning of an adhesive layer, and a thermal treatment is carried out to effect diffusion between the coating and the substrate.
At least two films are formed by presintering powders having different compositions from one another, and the said films are superposed on one another and on the said substrate with the interpositioning of adhesive layers, to obtain after the said thermal diffusion treatment a coating having a composition gradient.
The said coating is formed by a vapour phase physical deposition technique from a source that is itself obtained by bonding granules of the said powder.
The coating is formed by evaporation in an electron beam.
The said source comprises, apart from the constituent alloy of the powder, a ceramic element joined to the metal element by a sealing cement comprising a middle layer of mullite connected to the ceramic element by a transition layer whose chemical composition changes progressively, with continuity of crystalline structure, from the composition of mullite to that of the ceramic element, and also joined to the metal element by an intermediate layer containing mullite, silica and an aluminide of nickel and/or cobalt, at concentrations that vary progressively between the middle layer and the metal element.
The coating is formed by cathodic sputtering.
The powder can be used, inter alia, to form on a turbine machine casing an impermeable coating that can be abraded by rotating turbine blades.
In the process according to the invention the modifying element is most often deposited on a powder of MCrAlY or of CrAlY(Ta), from a chemical deposition bath containing ions of this element. An AMDRY 997 powder or an AMDRY 995 powder may for example be used as starting material. Alternatively a powder free of Ni and Co may be used as starting material and one and/or other of the the said elements may be deposited in a complementary stage preceding or following the deposition stage of the modifying element, which enables a powder to be obtained whose composition is adapted to that of the alloy to be protected.